Walton John C, Dai Wen, Curran Dennis P
EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, U.K.
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States.
J Org Chem. 2020 Mar 20;85(6):4248-4255. doi: 10.1021/acs.joc.9b03420. Epub 2020 Feb 27.
The boron-centered radicals derived from alkenyl -heterocyclic carbene (NHC)-boranes bearing ester substituents were recently found to ring close in 5-endo mode by addition to the oxygen atoms of the ester substituents. The inference from this was that NHC-boryl radicals might add intermolecularly to carbonyl-containing substrates. Several different NHC-boryl radicals were generated by H-atom abstraction from NHC-ligated trihydroborates. Electron paramagnetic resonance (EPR) spectroscopy proved that these did indeed add to the oxygen atoms of diaryl ketones with production of the corresponding bora-ketyl radicals. The same unusual regioselectivity of addition was observed with monoaryl ketones, but no bora-ketyls were observed with dialkyl ketones. Similarly, no bora-ketyl adduct radicals were observed with esters, even esters of benzoic acid. EPR spectroscopic evidence suggested that NHC-boryl radicals were also added to the O-atoms of aromatic aldehydes. Amine-boryl and phosphine-boryl radicals were also observed to add to the O-atom of benzophenone with production of the corresponding ketyl radicals.
最近发现，带有酯取代基的烯基-杂环卡宾（NHC）-硼烷衍生的硼中心自由基通过加成到酯取代基的氧原子上以5-内型模式闭环。由此推断，NHC-硼基自由基可能会分子间加成到含羰基的底物上。通过从NHC连接的三氢硼酸盐中夺取氢原子生成了几种不同的NHC-硼基自由基。电子顺磁共振（EPR）光谱证明，这些自由基确实加成到二芳基酮的氧原子上，生成了相应的硼酰基自由基。单芳基酮也观察到了相同的异常区域选择性加成，但二烷基酮未观察到硼酰基自由基。同样，酯类，甚至苯甲酸酯类也未观察到硼酰基加合物自由基。EPR光谱证据表明，NHC-硼基自由基也加成到芳族醛的O原子上。还观察到胺-硼基和膦-硼基自由基加成到二苯甲酮的O原子上，生成了相应的酰基自由基。
